A basic component of a printed circuit board is a dielectric layer having a sheet of copper foil bonded thereto. Through a subtractive process, that includes one or more etching steps, portions of the copper foil are etched away to leave a distinct pattern of conductive lines and formed elements on the surface of the dielectric layer. Multi-layer printed circuit boards are formed by stacking and joining two or more of the aforementioned dielectric layers having printed circuits thereon.
The trend, in recent years, has been to reduce the size of electronic components and provide printed circuit boards having multi-chip modules, etc. This results in a need to increase the number of components, i.e., surface mount components, provided on a printed circuit board. A key to providing a densely populated circuit board is to produce close and fine circuit patterns from the copper. The width and spacing of conductive paths on the printed circuit board are generally dictated by the thickness of the copper on the dielectric layer.
It has been proposed to use copper-coated polyimide components in forming printed circuits. The thickness of the copper on polyimide is generally significantly less than the thickness of traditional copper foil sheet. The thinner copper on the polyimide allows for finer and more closely spaced circuit lines in that the thinness of the copper layer reduces the etching time required to remove unwanted copper. In this respect, it is possible to use copper clad polyimide wherein the copper has a thickness as low as 0.1 .mu.m (1,000 .ANG.). The thinner copper on the polyimide also finds advantageous application in a semi-additive process. In a semi-additive process, the copper is masked to define a circuit pattern, and copper is plated onto the exposed pattern to build up a circuit. The mask material is removed and a "flash etch" removes the base copper on the polyimide leaving the built-up circuit on the polyimide. Thus, copper on polyimide finds advantageous application in both subtractive and semi-additive processes for forming printed circuits.
The use of copper-coated polyimide components in forming printed circuit boards or multi-layer laminates requires good adhesion between the polyimide side of the component and the inner core laminate to which it is attached. However, it is generally known that polyimide itself has relatively poor adhesion properties.
The present invention overcomes this and other problems, and provides a copper-coated polyimide component having improved adhesion with the inner core laminate.